Here is a guide on Chaos Fighters for iOs and Android written by PerfectLuck that tackles Stat Optimization.

To start, let's breakdown the stats.

Battle Stats: HP/SP/MinATK/MaxATK/SPD/HIT/EVA/BRK/DEF/CRT/RES
These attributes are what is factored in actual battle. In battle, these are the only statistics that matter. When you are reforging and changing your gear, your end goal is to affect these attributes.

Each type of equipment has its place on this spectrum (Weapon and Chest equipment hold two spaces). Basically all gloves will have an 8 in the ones digit--so 8, 18, 28, 38, 48, etc. So the equipment by level are as follows:

1: Weapon

2: Chest

3: Helm

4: Necklace

5: Shoes

6: Weapon

7: Chest

8: Glove

9: Legging

0: Ring

Equipment power go up by one every 5 levels. So the weapon/chest/helm/necklace/shoes of levels 41, 42, 43, 44, and 45 have the same obtainable stats and the weapon/chest/glove/legging/ring of levels 46, 47, 48, 49, and 50 have the same obtainable stats, but are higher than the ones from 41-45.

Only focus the level 46-50 power bracket to compare the stats you'll find on them. Focus on the bracket of levels 46-50. Since the maximum of a stat is the only standardized measurement you can get from these equips, Compare attributes using their maximum. Note that purple equipment have a higher maximum on their 4th attribute slot, so I am disregarding those. Here is the list of maximum obtainable stats for all attributes in equipment from level 46-50.

So most of these are straightforward, but probably the most pervasive question is...

STR or ATK? DEX or EVA or SPD? STA or HP or SP? Which fighter should I use!?
The main difference in fighters is their rate stats. What I mean by rate is the ratio at which they gain Primary Attributes based on STR/DEX/STA. These are denoted by the fighter page saying "Each R STR/DEX/STA gains ___". And yes, I will now be referring to these values as R.

First. Some of the stats offered by STR/DEX/STA are actually USELESS or IRRELEVANT.

STR is okay. It only offers ATK% which is very useful.

DEX offers both SPD and EVA, each at 1 per R. But let's compare it to the actual primary stats of SPD and EVA.

You have the chance, on an equip, to gain the following:
77 DEX
4.7 SPD
70 EVA

Regarding EVA gained by DEX, it is nearly useless. Even if you have a DEX R value of 7, which is amazing and nonexistent, 77 DEX will only give you 11 EVA, which is nothing compared to simply rolling the EVA stat at 70. Thus, all in all, increasing your DEX really only increases your SPD.

STA is easy. You will rarely run out of SP, so increasing SP is relatively useless.

So of the Secondary attributes, here's what's important about them.
STR: %ATK, DEX: SPD, STA: %HP

That said, the next question is at what point is the primary attribute better than the secondary? (i.e. at what point is 77 STR better than 4.7% ATK?)

This is completely dependent on what your R is for the fighter. So let's start at the middle line. At what point is 77 STR comparable or better than 4.7% ATK? 77/4.7 = 16.4

So if your fighter gains 1% ATK per 16 STR, or an R value of 16, then it is generally better for you to get STR over ATK%. Here's a quick table:

So if your fighter, for every 6 STR, gains 1% ATK (R = 6), you will actually be gaining 12.8% ATK from 77 STR, which is much higher than the maximum on the 1%ATK stat itself. Note that as your R goes down, the increases in the Primary Stat you gain get larger and larger. It is better to have an R of 6 over 7 than an R of 12 over 13.

Note that this does not only apply to STR:%ATK. The numbers for DEX:SPD and STA:%HP are all the same -- 77:4.7.

So what does this mean for choosing a fighter? Choose a stat you want to base your fighting style on and focus on getting that stat as low as possible, whether it's %ATK, SPD, or %HP. Here's a list of fighters with very low R, and thus very high gains on their respective stats.